Chapter 1Where the picture comes from
The net's got two granddaddies — one a war charm, one a sermon — and a monk who built it out of mirrors to show an empress what he meant.
First, it was a weapon
The oldest net of Indra shows up in the Atharva Veda, a book of Sanskrit charms and spells from India near three thousand years back. Book eight has a charm for whippin' an enemy army: throw Indra's net over 'em and tangle 'em up. Then it goes big and says this whole great world is the net of great Indra1. So from the start the net was two things at once — a trap you throw, and the whole world you're standin' in.
The word stuck around. In later Sanskrit, indrajāla — Indra's net — came to mean a conjurin' trick, magic, an illusion2. Street magicians were doin' indrajāla. Hold on to that; it comes back on the readings page.
Then it was a sermon
A long Buddhist scripture called the Flower Garland Sūtra (the Avataṃsaka) fills heaven with jewels and lights that shine into each other3. In China, a school of Buddhism grew up around that book called Huayan (say it “hwah-yen”; Kegon in Japan, Hwaeom in Korea). One of its founders, a monk named Dushun, around the year 600, took Indra's net and made it a teachin' picture: every jewel reflects every other jewel, and every reflection holds all the others too, without end45.
His point was not about jewelry. It was that every thing is like that — it holds all the rest inside it, and it is held inside all the rest.
Then a monk built it
A generation later, a Huayan teacher named Fazang had to explain all this to Empress Wu Zetian, who was busy and wanted to see it, not hear it. So the story goes, he set up mirrors on all eight sides of a room, plus one overhead and one underfoot, put a Buddha statue in the middle, and lit a lamp. Step in, and there's the statue in every mirror, and in every mirror inside every mirror, on out past countin'76.
You can draw his room with plain arithmetic. Squash it flat to a square with four mirror walls, then do the trick every pool shark knows: instead of followin' the light as it bounces, unfold the room. Flip it over each wall like a page. Then flip them copies. The bouncin' path turns into a straight line out through a grid of rooms, and every lamp you see in the mirrors sits in one of those rooms.
Readin' it: count the lamps you can see that took N bounces or fewer. One bounce gives you 5: the lamp and its four reflections. Four bounces, 41. Ten, 221. A hundred, 20,201. Fazang had ten mirrors, not four, and in three directions, not two — the count climbs a whole lot faster, and it don't stop.
Why the lamps get dimmer
No mirror sends back all the light it gets. A good household mirror keeps somethin' like nine parts in ten, so a lamp ten bounces deep is down to about a third. The picture above dims each bounce to 82%. The Huayan jewels are perfect mirrors, so in the story they don't dim — every reflection is as bright as the thing itself. That's one place the story's net and a room you can build part company.
And here's a thing Fazang didn't have to worry about: light takes time. Each bounce is a few billionths of a second late, so every lamp in the mirrors is a lamp from a hair in the past. How fast light goes, and why it won't go faster, is its own 'splainer: Goin' Fast.